Adipic acid is widely used in the manufacture of polyesters, polyamides, plasticizers and the like. Conventionally adipic acid is known to be produced by two step oxidation of cyclohexane. In the first step KA oil is produced from cyclohexane by air oxidation which is then oxidized by nitric acid to dicarboxylic acids such as adipic acid, glutaric acid and succinic acid. This process suffers from various disadvantages, especially relating to low conversion per pass of the hydrocarbon in the first step (3-8%), leading to large recycle; use of nitric acid in the second step leading to NOx pollution; and formation of byproducts like lactones which pose difficulties in the downstream use of adipic acid.
In order to overcome drawbacks in the two step oxidation process, one step oxidation of cyclohexane has been tried over the years. Air is used as an oxidant in one step oxidation of cyclohexane. The catalysts mostly used in the oxidation of cyclohexane are cobalt and its mixture with other metals like iron. In the conventional methods, in-situ activation of the catalyst from cobaltous to cobaltic state is carried out with the help of activators like aldehydes and ketones.
The earlier known patents have divulged the use of any oxygen containing gas, however, the known art does not bring-out the differences in selectivity to adipic acid and conversion of CH while using pure oxygen instead of any oxygen containing gas. The prior art has also not been able to achieve a higher selectivity to give better yields using high purity oxygen versus use of air or dilute oxygen as conventionally used.
The direct oxidation of cyclohexane to adipic acid is a process that has attracted much attention on account of the obvious advantages there would be in avoiding the nitric acid oxidation of KA-oil and its associated nitric acid handling activities. Processes that have been proposed for preparing dibasic acids without the use of nitric acid include air oxidation of saturated cyclic hydrocarbons and/or corresponding cyclic ketones and/or-alcohols. For example, U.S. Pat. No. 3,390,174 and British Patent 1,304,855 disclose processes requiring mixtures of two or more of these components. However, many such air oxidation processes are multi-step processes and do not provide the required high yield as well as high selectivity.
Single-step direct air oxidation processes for the production of dibasic acids have also been proposed and used in the prior art. However, previously known one-step processes have been attended by poor selectivities, low production rates, costly separation steps and relatively rapid deactivation of the catalyst during its recycling. For example, U.S. Pat. No. 2,223,493 discloses a process for the oxidation of cyclic hydrocarbons to corresponding diacids, in a liquid phase generally containing acetic acid at a temperature of at least 60.degree. C., using a gas containing oxygen and in the presence of an oxidation catalyst such as cobalt compound. The selectivities reported in this patent are as low as 46 to 49 mole percent. Furthermore, the patent fails to address the issues of recycling the catalyst and the activity which a catalyst that has been recycled one or more times would have.
U.S. Pat. No. 2,589,648 describes a single-step oxidation process wherein acetone is used in the place of acetic acid as solvent.
U.S. Pat. No. 3,231,608 describes a similar process, suggesting that molar ratios of solvent to saturated cyclic hydrocarbon in the range of 1:5:1 to 7:1 (or more) are suitable but that molar ratios below or above this range give unsatisfactory results.
U.S. Pat. No. 4,032,569 teaches a process for converting cyclohexane to adipic acid which involves oxidizing cyclohexane with molecular oxygen in the presence of critical amounts of cobaltic ions, which is mentioned to be in the range of 25 to 150 millimols per mole of cyclohexane charged. Catalyst activation is in situ during the initial induction period. It is observed in this patent that either air or molecular oxygen can be used for the oxidation. No effect of gas phase concentration of oxygen is studied, but, the total pressure is mentioned to be higher than 11 kg/cm.sup.2. Adipic acid, glutaric acid and succinic acid are the main products of the reaction with very low concentration of others. The analysis method, however, is not specified.
U.S. Pat. No. 4,263,453 is a continuation of the above work, with a modification in the procedure that water is added to the reaction mixture in the initial stages to enhance the selectivity to adipic acid. It is claimed that the addition of water improves the yield of adipic acid. However the induction period increases several fold thereby decreasing the rate of production severely.
U.S. Pat. No. 4,902,827 discloses a process for the production of adipic acid from cyclohexane in the presence of cobalt and zirconium and/or hafnium catalyst using air.
U.S. Pat. No. 5,221,800 also discloses a process for producing adipic acid from cyclohexane but with intermittent addition of water. It is claimed that water, if present during the induction period, depletes the concentration of free radicals which are so essential for catalyst activation It is shown that water addition after the induction period is more advantageous and results in a selectivity of about 88%. However this selectivity calculation is based only on the identifiable compounds. The true selectivity with all other compounds considered is not given.
U.S. Pat. No. 5,321,157 revisits the cobalt catalysed oxidation. All previous work on one-step oxidation is argued in this patent to be inefficient as they employ low initial concentration of cyclohexane and higher reaction times. They claim that oxidation of high concentrations of cyclohexane at low conversion levels provides advantageous chemical and economic results. Low conversions are achieved by using low concentrations of catalyst. Use of oxygen gas or oxygen containing gas is recommended and the conversions are restricted to below 30%. The product mixture is analysed in gas chromatography after esterification with excess methanol. This is claimed that with this analysis, selectivity of adipic acid is obtained to be about 80 to 88%. In this work also oxygen or any oxygen containing gas is claimed to produce the desired results.
Patent Application WO-A-94/07834 and U.S. Pat. No. 5,463,119 disclose a process which develops the stage for the purification of the final mixture. This treatment consists in separating the diacid formed, by cooling the mixture in order to bring about precipitation of the diacid, and in separating by filtration the diacid from two liquid phases, a non-polar one which is recycled, and a polar one which is also recycled after an optional hydrolysis and a separation of an additional amount of diacid. Furthermore, it provides significant information on removing the aliphatic dibasic acid (preferably as a precipitate collected by filtration or centrifugation) and recycling intermediates, post-oxidation components, and derivatives thereof remaining after removal of the aliphatic dibasic acid.
U.S. Pat. No. 5,756,837 is similar to this patent but discloses a process for recycling a catalyst containing cobalt including treating a reaction mixture obtained during the direct oxidation of cyclohexane to adipic acid by extracting at least some of the glutaric acid and succinic acid which are formed in the reaction.
U.S. Pat. No. 5,547,905, a patent by the same applicant as of the present invention, describes a process for adipic acid production by one-step oxidation in the presence of a novel Co-Fe catalyst. In this patent, a new catalyst activation step is disclosed and use of pure oxygen as oxidant is mentioned. In this work conversion of cyclohexane is taken upto 70% in about 4 to 6 hours. Average selectivity to adipic acid is about 75%. This patent also shows that the Co-Fe catalyst can be used successfully for the oxidation of other hydrocarbon like xylenes, toluenes and cyclopentane to their corresponding dicarboxylic acid.